Uniform Al2O3 Research Articles

Atomic-Layer-Deposited Al2O3 as Gate Dielectrics for Graphene-Based Devices

We present a facile route to deposit a uniform Al2O3 layer on a highly oriented pyrolytic graphite (HOPG) surface using atomic layer deposition (ALD). Al2O3 layers deposited from TMA (trimethylaluminum)/ H2O chemistry showed selective deposition only on step edges. However, TMA/O3 chemistry resulted in the deposition of Al2O3 layers on basal planes of HOPG, which has a chemically inert surface. An O3-pretreatement followed by Al2O3 deposition using TMA/O3 chemistry produced conformal and uniform Al2O3 dielectric layers with a small RMS roughness of ~ 0.2 nm. This suggests that O3-pretreatement prior to ALD deposition makes the chemically inert HOPG surface reactive toward ALD precursors, which leads to the desired two-dimensional growth mode. High-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy revealed that this O3 process used for Al2O3 deposition does not introduce a significant defect concentration to the top graphene layer. The dielectric constant of the deposited Al2O3 film on top of the HOPG surface was found to be ~ 9 from C-V measurements.

Ultrafast metal-insulator varistors based on tunable Al2O3 tunnel junctions

Ultrafast metal-insulator varistors have been fabricated using atomic layer deposition. A high-density matrix of micron-sized spherical Ni particles conformally coated with ∼7.5–22nm Al2O3 films exhibited transient response times (∼0.3ns), capacitances (∼45pF), leakage currents (∼33pA), and nonlinearities (α∼380) which were all markedly improved over conventional metal oxide varistors. These characteristics result from the Fowler–Nordheim tunneling of electrons through uniform Al2O3 tunnel junctions separating adjacent particles within the matrix.

Throwing Power during Electrophoretic Deposition

The deposit can induce an extra potential drop near the electrode, depending on the suspension composition. This can result in a levelling off of the deposition rate in a constant-voltage deposition process. The magnitude of the extra voltage drop determines the uniformity of the deposit as function of the uniformity of the electric field present at the deposition electrode. It was experimentally proven that a uniform Al2O3 coating thickness was obtained in a non-homogeneous electrical field in ethanol with addition of HNO3, while the coating thickness varied uniformly with the E-field strength for a MEK with n-butylamine based suspension. The uniformity of the coating deposited from these suspensions was related to the measured potential drop over the deposit during electrophoretic deposition.

Fabrication of All-Solid-State Lithium Polymer Secondary Batteries Using Al2O3-Coated LiCoO2

Al2O3-coated LiCoO2 cathode particles were prepared by a spray coating method, and a uniform nanoscaled Al2O3 layer was formed on the surface of LiCoO2. The capacity retention in a LiCoO2 cathode material for the all-solid-state lithium polymer secondary batteries was improved by Al2O3 coating on LiCoO2 as well as liquid-type lithium batteries.

Surface study and thickness control of thin Al2O3 film on Cu–9%Al(111) single crystal

We were successful in growing a uniform flat Al2O3 film on the Cu–9%Al(111) surface using the improved cleaning process, low ion energy and short time sputtering. The growth of ultra-thin film of Al2O3 on Cu–9%Al was investigated using Auger electron spectroscopy (AES) and a scanning electron microscope (SEM). The Al2O3 film whose maximum thickness was about 4.0nm grew uniformly on the Cu–9%Al surface. The Al and O KLL Auger peaks of Al2O3 film shifted toward low kinetic energy, and the shifts were related to Schottky barrier formation and band bending at the Al2O3/Cu–9%Al interface. The thickness of Al2O3 film on the Cu–9%Al surface was controlled by the oxygen exposure.

Metal oxide nanoarchitectures for environmental sensing.

Metal oxide materials are widely used for gas sensing. Capable of operating at elevated temperatures and in harsh environments, they are mechanically robust and relatively inexpensive and offer exquisite sensing capabilities, the performance of which is dependent upon the nanoscale morphology. In this paper we first review different routes for the fabrication of metal oxide nanoarchitectures useful to sensing applications, including mesoporous thin films, nanowires, and nanotubes. Two sensor test cases are then presented. The first case examines the use of highly uniform nanoporous Al2O3 for humidity sensing; we find that such materials can be successfully used as a wide-range humidity sensor. The second test case examines the use of TiO2 nanotubes for hydrogen sensing. Going from a nitrogen atmosphere to one containing 1000 ppm of hydrogen, at 290 degrees C, 22-nm-diameter titania nanotubes demonstrate a 10(4) change in measured resistance with no measurement hysteresis.

Effect of oxygen pressure on the structure and thermal stability of ultrathin Al2O3 films on Si(001)

Al 2 O 3 /Si(001) surfaces and interfaces were investigated using scanning reflection electron microscopy, reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, and Auger electron spectroscopy. A uniform, stoichiometric and ultrathin Al2O3 film of about 0.6 nm was grown on an atomically flat Si(001)-2×1 surface, and the resulting Al2O3/Si(001) interface was atomically abrupt. An intentional reoxidation of the Al2O3/Si(001) system under low oxygen pressure (2×10−6, 5×10−6, and 2×10−5 Torr O2) showed that the ultrathin Al2O3 film stoichiometry and the interface abruptness were maintained with progress in reoxidation time. Furthermore, the film and the interface showed no degradation under low-pressure reoxidation at various temperatures (400–750 °C). A high-pressure reoxidation of the Al2O3/Si(001) system at 5×10−5 Torr O2 resulted in the formation of an interfacial SiO2 layer which grew in a layer-by-layer mode with atomic-scale uniformity and had an atomically abrupt interface with Si(001) substrate up to 700 °C. Additionally, a very weak temperature dependence of the growth of interfacial SiO2 was observed. A high-pressure reoxidation at 750 °C led to the formation of crystalline ultrathin Al2O3 film and also caused degradation of the film by formation of SiO2 in the near-surface region, where a slight decrease in the Al2O3 film thickness was observed. This was attributed to the formation of interstitial Si in the interfacial SiO2 layer and the subsequent mobility of Si and Al under this growth condition. Under low-pressure reoxidation, the Si and Al were immobile because of the absence of an interfacial SiO2 layer at the Al2O3/Si(001) interface. These results indicate that the oxygen pressure of the ambience plays an important role in the oxidation of the Al2O3/Si(001) interface, and the mobility, transport, and chemical reactions at various oxidation temperatures (400–750 °C).

RF Packaging for Space Applications: from Micropackage to SOP – “System On a Package”

Al2O3–cyanate ester hybrid thick films were fabricated for 3-D integrated substrates by resin infiltration into Al2O3 porous thick films fabricated by an aerosol deposition (AD) process at room temperature. Although the AD process has been proposed as a low temperature process for fabricating dense ceramic thick films using submicron ceramic particles as starting powders, we carried out fabrication of porous Al2O3 thick films using nano-powders as the starting powder—employing the phenomenon that this fabricates only pressed compacts. The surface roughness and packing density of the fabricated porous Al2O3 thick films could be controlled by carrier gas flow, as a result, uniform porous Al2O3 thick films were fabricated on glass and Cu substrates at room temperature by the AD. There was no peak shift or peak broadening of x-ray diffraction patterns in the porous Al2O3 thick films compared to dense Al2O3 thick films made with submicron-starting powders. Al2O3–cyanate ester hybrid thick films were fabricated by resin infiltration into the porous Al2O3 thick films, and heated to 280 °C for 5 h to cure the cyanate ester. The polymerization of cyanate ester monomers was confirmed from Fourier transform infrared spectroscopy after curing. The microstructures of Al2O3–cyanate ester hybrid thick films were observed by scanning electron microscope after focused ion-beam milling. The relative permittivity and loss tangent of Al2O3–cyanate ester hybrid thick films were 6.7 at 1 MHz and 0.001, respectively. The Al2O3 content in the hybrid thick films was calculated using Hashin–Shtrikman bound theory with 3-D electrostatic simulations, and the calculated content was 72 vol.% near a face-centered cubic structure.

Study of ultrathin Al2O3/Si(001) interfaces by using scanning reflection electron microscopy and x-ray photoelectron spectroscopy

Al 2 O 3 /Si (001) interfaces were investigated using scanning reflection electron microscopy and x-ray photoelectron spectroscopy. A uniform and stoichiometric ultrathin Al2O3 film of ∼0.6 nm was grown on an atomically flat Si(001)-2×1 surface, and the resulting Al2O3/Si(001) interface was atomically abrupt. Furthermore, an intentional high-pressure oxidation shows that we can grow Si oxide at the Al2O3/Si(001) interface with atomic-scale uniformity as this oxidation proceeds in a layer-by-layer manner. The resulting Si oxide/Si(001) interface was also atomically abrupt. In addition, the rate of oxidation of Si at the Al2O3/Si(001) interface depends strongly on the O2 pressure.

Formation of Very Thin Epitaxial Al2O3 Pre-layer with Very Smooth Surface on Si (111) Using a Protective Oxide Layer

We propose the formation of an Al2O3 pre-layer using a protective Si-oxide layer and an Al layer. Deposition of a thin layer of aluminum onto a Si surface covered with a thin Si-oxide layer and annealing at 800°C led to the growth of Al2O3 layers on Si (111). A very smooth and uniform Al2O3 (111) pre-layer film was epitaxially grown on a Si (111) substrate. The surface of the γ-Al2O3 film grown on the Al2O3 pre-layer was very smooth with a Z range of ∼3 nm. However, the surface grown without the Al2O3 pre-layer had numerous convex structures concerning with SiOx clusters with a Z range of ∼10 nm. Etching of the Si substrate by N2O gas could be avoided in the initial growth stage by the Al2O3 pre-layer. It was confirmed that the Al2O3 pre-layer was effective in improving the surface morphology of the very thin γ-Al2O3 films.

Thickness determination of uniform overlayers on rough substrates: A comparison of calculations for Al2O3/Al to x-ray photoelectron spectroscopy and atomic force microscopy experiments on technical aluminum foils

Angle dependent x-ray photoelectron spectroscopy (XPS) is often used to determine the thickness of thin overlayers by means of a model assuming a perfectly flat substrate. In this article we analyze the consequences of neglecting roughness effects for a uniform Al2O3 overlayer on a rough Al substrate, through the application of a Monte Carlo algorithm to rough geometries generated by a method for simulation of random fractals. The main result is that the error in calculated thickness—which depends strongly on analyzing angle—nearly vanishes at off-axis angles close to 35°. We apply these findings to previously published XPS data on technical aluminum foils and compare the outcome with an atomic force microscopy based roughness analysis of these foils.

溶融アルミニウムとAl<SUB>2</SUB>O<SUB>3</SUB>-SiO<SUB>2</SUB>系基盤の濡れ性および反応

The wettability and the reaction between molten Al and Al2O3-SiO2 substrates were measured by the sessile drop method in a vacuum of 2×10−3 Pa at temperatures from 973 to 1173 K. The contact angles of molten Al to Al2O3 and Al2O3-SiO2 substrates decreased from about 110° to 90° as the temperature rose from 973 to 1173 K. The reaction between molten Al and NC mullite, which consisted of mullite and vitreous SiO2, produced a mixed layer of Al2O3 and mullite. The reaction layer grew in proportion to the holding time at 973 K. The linear growth was also recognized until 300 s at 1073 K, and the growth rate was controlled by the chemical reactions:3SiO2+4Al=3Si+2Al2O33(3Al2O3·2SiO2)+8Al=6Si+13Al2O3On the contrary, at 1173 K or after 300 s at 1073 K, the reaction layer grew parabolically with the increase in time. The growth rate was controlled by the diffusion of Al and Si through the reaction layer.The uniform Al2O3 layer formed between molten Al and mullite substrate at 1173 K after an incubation period. The growth rate was also governed by the diffusion of Al and Si.The Si pickup by Al drop was estimated by the thickness of the reaction layer between molten Al and NC mullite or mullite substrate.

Development of Sputter-Deposited Pt/Al2O3 Selective Absorber Coatings

The Pt/Al2O3 cermet system is extremely stable against oxidation. Therefore, effective high temperature selective absorber coatings can be formed by depositing Pt/Al2O3 cermets onto Pt low emittance base layers. Three Pt/Al2O3 cermet configurations have been investigated for photothermal receiver applications: (1) cermets with graded Pt composition and Al2O3 antireflection layers; (2) cermets with uniform Pt compositions and Al2O3 antireflection layers; and (3) Al2O3-M-Al2O3 multilayer coatings where the M-layer is a Pt/Al2O3 mixture containing about 20 volume percent Al2O3. The coatings were deposited by cosputtering from Pt and Al2O3 sources. Both direct rf sputtering from an alumina target and Ar-O2 dc reactive sputtering from an aluminum target were investigated as methods for depositing the Al2O3 components of the cermet and the antireflection layers. Cr, Mo, Ta, W, and ZrB2 were investigated as alternative low emittance base layer materials. Most of the coatings were deposited onto flat plates of glass, although type 316 stainless steel, type 1020 low carbon steel, and Incoloy 800 substrates were also examined. Hemispherical absorptances as high as 0.97, with room temperature emittances in the 0.06 to 0.08 range, were achieved. The optical properties of the coatings with rf-sputtered Al2O3, and Pt low emittance base layers were stable at 600° C in air for as long as 2000 hr, consistent with data for evaporated Pt/Al2O3 coatings reported by Cornell University. The coatings with reactive sputtered aluminum oxide, or alternative base layers such as Mo, exhibited somewhat reduced thermal stability but should be adequate for intermediate temperature applications (up to 400° C in air).

Cognitive outcomes of preterms with RDS

In situ composites of TiAl reinforced with Al2O3 particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb2O5 by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al2O3, NbAl3, as well as small amounts of the Ti3Al phase. The in situ formed fine Al2O3 particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al2O3 distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb2O5 content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m1/2, respectively, when the Nb2O5 content reaches 6.62 wt.%. The strengthening mechanism was also discussed.